1. Field of the Invention
The present invention relates to optical wavelength-division multiplexing (WDM) networks. More specifically, the present invention relates to a method and an apparatus for guaranteeing a stated failure-recovery time in an optical WDM network.
2. Related Art
Wavelength-division multiplexing (WDM) technology enables an optical fiber to support over one hundred wavelength channels, each of which can operate at a bandwidth of several gigabits per second (Gbps). A failure in such an optical network, e.g., a fiber cut or fiber conduit cut, can lead to the loss of a huge amount of data (several terabits per second (Tbps) to several petabits per second (Pbps)). Therefore, efficient fault-management schemes are extremely important for WDM optical networks to recover from such network failures. Being able to guarantee a maximum failure-recovery time is an extremely critical issue for the network operator in order to minimize the loss of data (and revenue) caused by the failure.
In a conventional network that supports voice traffic, a network failure needs to be recovered within 50 ms. This is due to the persistence of hearing of the human ear. If the failure is recovered within 50 ms, voice users will not notice any serious interruption during their conversation. Because existing networks were developed to support voice traffic, the failure-recovery time of 50 ms is presently a well-established standard.
Today's networks are mainly constructed as synchronous optical network/WDM (SONET/WDM) interconnected-ring-topology networks. SONET networks provide a self-healing mechanism, in which automatic protection switching (APS) is used to protect the traffic. By limiting the size of rings in a SONET network, APS can be used to recover a network failure within 50 ms.
In a SONET/WDM ring, 100% spare capacity is pre-reserved to provide protection capacity. However, this is a very inefficient strategy since bandwidth is a precious resource. As optical networks evolve from interconnected-ring topologies to arbitrary-mesh topologies, the optical cross-connects (OXCs) are expected to be mesh-enabled. Hence, they should be able to provide efficient and reliable protection schemes to replace APS in SONET ring networks.
Currently, many systems vendors and network operators consider end-to-end path protection to be the main protection scheme to be used in WDM mesh networks. In end-to-end path protection, a pair of link-disjoint paths is found for a connection request: one for the primary (working) path, and the other for backup (protection) path.
In WDM mesh protection, the failure-recovery time is determined by three main factors:                failure detection time (FDT): the time needed for the nodes around the failure point to detect the failure;        failure notification time (FNT): the time needed to notify the source node of the connection that a failure has occurred;        restoration time (RT): the time needed for dynamic discovery of backup resources; and        protection switching and re-signaling time (PST): the time needed to activate the backup path, and to switch the traffic from the primary path to the backup path.Due to current technology limitations (message processing time, OXC switching time, etc.) and physical constraints (e.g., the fiber propagation delay imposed by the speed of light), it is difficult to guarantee a specific failure-recovery time in WDM mesh networks, especially when the requirement is in the millisecond scale. While a 50 ms failure-recovery time is acceptable for voice transmission, much data can be lost in that amount of time. Hence, many users of WDM networks desire shorter failure-recovery times and are willing to pay a premium for the service, while other users are willing to accept longer failure-recovery times for a reduced fee.        
Hence, what is needed is a method and an apparatus for guaranteeing a stated failure-recovery time in a WDM mesh network.